1. Field of the Invention
The present invention relates to a metal halide lamp, and particularly to a lamp which is suitably used for cultivation of plants.
2. Description of the Related Art
Much attention has been recently paid to plant factories because the plant factories have a merit that various kinds of plants can be stably produced and supplied with high quality irrespective of weathers and surrounding environments.
The types of plant factories are roughly classified into a sunlight using type in which plants are cultivated by basically using sunlight as main light and using artificial light as auxiliary light, and a perfectly artificial light type in which plants are cultivated under an enclosed environment with using no sunlight. Plant factories which cultivate plants under the enclosed environment are also called as enclosed type plant factories, and they can perform plant cultivation which is never influenced by weathers and surrounding environments because cultivation conditions such as an irradiation light amount, etc. for plant cultivation can be strictly controlled.
Furthermore, it has been recently known that it is effective from the viewpoint of photosynthesis of plants to apply light of three colors of red light, green light and blue light to plants with good balance when the plants are cultivated with artificial light (see JP-B-3-49530, for example). A metal halide lamp has been suitably used as a light source of artificial light (see JP-A-2003-339236 and JP-A-2009-87602, for example).
Leaf vegetables such as lettuce, herb, etc. have been hitherto mainly cultivated in plant factories, however, root vegetables whose roots or rhizomes are edible (such as carrot, radish, etc.), fruit vegetables whose berries are edible (such as cucumber, tomato, etc.) and cereal such as rice, etc. have been also recently cultivated.
The inventors of this application have knowledge that growth of plants varies in accordance with a sunshine condition subsequently to a rice seeding period, and there is a problem that cultivation using a conventional metal halide lamp as a light source for plant cultivation is lower in efficiency than cultivation based on sunshine.